Remove fat from steak, cut into strips salt and pepper. Melt the butter in a saucepan, add the meat and saute quickly tossing strips around to brown evenly. Add diced onions cook until clear.
At this point you can add the beef, onions, and the beef broth to your crock pot. Make sure the beef is covered with the liquid. If the broth is not enough add enough water until covered. Put the lid on the crock pot turn the temp to low and walk out the door.
When you get home stir in the mustard. Let cook 5 minutes more in the crock pot. Just before serving stir in the flour to thicken broth. Add sour cream, salt to taste. Serive over steamed rice or egg noodles.
This is easy to do and the crock pot does all the work. The uncrock pot way is to bring the beef strips and the beef broth to a boil. Turn heat down. Stir in the mustard. Cover and simmer for 1 hour until meat is tender. This is better the next day when the flavors get better tasting.